There are many examples of radio communications systems in which data is communicated using Orthogonal Frequency Division Multiplexing (OFDM). Systems which have been arranged to operate in accordance with Digital Video Broadcasting (DVB) standards for example, use OFDM. OFDM can be generally described as providing K narrow band sub-carriers (where K is an integer) which are modulated in parallel, each sub-carrier communicating a modulated data symbol such as Quadrature Amplitude Modulated (QAM) symbol or Quadrature Phase-shift Keying (QPSK) symbol. The modulation of the sub-carriers is formed in the frequency domain and transformed into the time domain for transmission. Since the data symbols are communicated in parallel on the sub-carriers, the same modulated symbols may be communicated on each sub-carrier for an extended period, which can be longer than a coherence time of the radio channel. The sub-carriers are modulated in parallel contemporaneously, so that in combination the modulated carriers form an OFDM symbol. The OFDM symbol therefore comprises a plurality of sub-carriers each of which has been modulated contemporaneously with different modulation symbols.
To facilitate detection and recovery of the data at the receiver, the OFDM symbol can include pilot sub-carriers, which communicate data-symbols known to the receiver. The pilot sub-carriers provide a phase and timing reference, which can be used to estimate a channel transfer function of a channel through which the OFDM symbol has passed, to facilitate detection and recovery of the data symbols at the receiver. In some examples, the OFDM symbols include both Continuous Pilot (CP) carriers which remain at the same relative frequency position in the OFDM symbol and Scattered Pilots (SP). The SPs change their relative position in the OFDM symbol between successive symbols, providing a facility for estimating the impulse response of the channel more accurately with reduced redundancy.
In order to reduce the effects of the channel through which the OFDM symbols have passed so that a receiver can detect and recover data to be communicated, it is necessary to generate an estimate of the channel transfer function at each of the sub-carrier positions in the OFDM symbol. Although scattered pilots and continuous pilots are provided in the OFDM symbols, in some examples it is preferable to be able to generate an estimate of the channel transfer function at each of the sub-carrier positions within the OFDM symbols, without having to store pilot symbols from previously received OFDM symbols, or having to wait until enough OFDM symbols have been received to provide a pilot at every sub-carrier position.